This invention pertains generally to frequency discriminators and particularly to circuitry of such type that is useful in a radar receiver wherein the frequency of an oscillator must be changed automatically to track the frequency of a received signal.
It is well known in the art to provide a frequency discriminator in an automatic frequency control (AFC) circuit in a radar receiver. Thus, many different AFC circuits have been designed on the idea that the frequency of a received signal relative to a known frequency may be determined to form a frequency control signal for an oscillator desired to be controlled. The signal out of the oscillator then may be used as the reference signal in a local oscillator to effect the desired automatic frequency control.
In the typical frequency discriminator used for the foregoing purpose, the received signal (or more precisely, the intermediate frequency analog of the received signal) is divided and passed through two channels wherein the difference between the phase shifts imparted on the divided parts of the received signal is related to the difference between the frequency of the received signal and a known frequency corresponding to the zero crossing of the discriminator. Therefore, by impressing the signals out of the two channels on a phase detector, a bipolar control signal may be produced to adjust the frequency of the oscillator until the bipolar control signal is nulled. Unfortunately, however, the inherent qualities of the phase shifting elements in the two channels cause the characteristic curve of any known frequency discriminator to assume the familiar "S" shape, which in turn means that, in addition to a desired frequency at which the bipolar control signal is properly nulled, there are so-called "image frequencies" at which the bipolar control signal may be improperly nulled.
In practice there are several other problems encountered when a conventional frequency discriminator is used in a frequency lock loop. Primary among these is the problem of spurious responses of the discriminator to out-of-band input signals. That is to say, because the characteristic curve of any known discriminator only approximates an "S" shape, (meaning that the discriminator may have spurious responses to in-band signals and may not be zero for all out-of-band signals) it is possible for the frequency lock loop to lock to one of such spurious responses. Thus, for example, if the spurious response were at, say, the 10 millivolt level and the discriminator were utilized in a frequency control loop having a gain of 100, then a bipolar control signal having a magnitude of 1 volt would be provided to the oscillator intended to be controlled. A control signal of such magnitude would be effective to tune the oscillator across an appreciable portion of its operating bandwidth with the unwanted result that a "lock" could be made on spurious response of the discriminator.
A second problem associated with the use of conventional frequency discriminators derives from the fact that any such discriminator is, per se, a narrowband element. This means that in many applications, where the frequency of a received signal may vary within a relatively wide band of frequencies, auxiliary means must be provided to attain a frequency lock on any received signal. The added complexity of any auxiliary means, along with the requirement that time be taken to search a relatively wide spectrum, is, to date, an accepted limitation of known frequency discriminators.
Still another problem associated with the use of conventional frequency discriminators derives from the fact that, with a phase detector as the sensing element, it is necessary that the amplitudes of the signals out of the two channels be substantially equal for optimum operation of the discriminator.